This definition is useful:
Somatisation and Functional Symptoms
Some doctors prefer to use the term functional when no known physical cause can be found for a physical symptom. A functional symptom means: a function of the body is faulty (for example, there may be pain or diarrhoea), but we dont know the cause. The cause may be due to mental factors (somatisation), physical factors not yet discovered, or a combination of both. Another term which is sometimes used for such symptoms is medically unexplained symptoms. Source:
here
This webpage summarizes most of the terms you have listed, Bob:
Textbook of Psychiatry/Somatoform Disorders
Wessely's done a few papers equating neurasthenia to ME. He's also equated it with burnout, as do many Dutch papers.
I'd suggest that this equating ME/CFS to burnout and neurasthenia may be correct — except that the cause of these illnesses is very likely an infectious one. I think that high fliers that suddenly got burnout, and quit their jobs, picked up a virus that entered their brain, and caused the burnout.
Highly paid high fliers in the 1980s (the decade of "yuppie flu") often followed a high living lifestyle of sex, drugs, and rock'n'roll. The drugs, and rock'n'roll probably were relatively harmless, but the pursuit of sex and amorous encounters is I suggest is where they most likely picked up a respiratory virus that then precipitated their burnout. I caught the virus that led to my ME/CFS from kissing on a date.
Burnout is always blamed on stress at work, but in fact when you are healthy, this stress is invigorating and exciting. Only once you develop burnout from an infectious cause does everything suddenly appear stressful.
"False illness beliefs." I'm not sure of the exact origin or "official" nature of this term, but it does show up quite a bit.
Wessely's (completely crazy) view is that ME/CFS and related conditions are simply caused by a person having
false illness beliefs. Wessely thinks that ME/CFS patients have inadvertently started to believe they are ill, and because of this belief, they then actually become ill. In other words, Wessely says there is no biological cause to ME/CFS, just I psychogenic cause: namely that some aberrant beliefs you hold about being ill actually make you sick.
Of course there is no evidence for such psychogenic causes whatsoever. Nobody has ever succeeded in planting a belief about being ill into a person's head, and then having that person come down with the illness. This never happens.
I wouldn't mind if Wessely kept his ideas to himself, or confined them to within his circle of unscientific psychologist friends, otherwise known as the Wessely School. But the problem is that Wessely School psychiatrists have advised the UK government, and so his views have become widely adopted, much to the detriment of the more scientific and evidence-based views on ME/CFS, such as the viral etiologies of ME/CFS.